'missing xobject' in simple test file

I get a 'missing Xobject Im1' error when I try to read the test PDF file below, using Acrobat 8.1.3 (and others), but it works when I read it with Ghostscript. Why is this?

Regards,
Terry

%PDF-1.7

1 0 obj
<<
/Type /Catalog
/Outlines 2 0 R
/Pages 3 0 R
>>
endobj

2 0 obj
<<
/Type /Outlines
/Count 0
>>
endobj

3 0 obj
<<
/Type /Pages
/Kids [4 0 R]
/Count 1
>>
endobj

4 0 obj
<<
/Type /Page
/Parent 3 0 R
/MediaBox [0 0 612 792]
/Contents 5 0 R
/Resources
<<
/ProcSet [/PDF /ImageB]
/XObject << /Im1 6 0 R >>
>>
>>
endobj

5 0 obj
<</Length 35>>
stream
q
050 0 0 050 010 010 cm
/Im1 Do
Q
endstream
endobj

6 0 obj
<<
/Subtype /Image
/Width 64
/Height 16
/ColorSpace /DeviceGray
/BitsPerComponent 1
/Length 160
/Filter /ASCIIHexDecode
>>
stream
f0f0f0f0
f0f0f0f0
f0f0f0f0
f0f0f0f0
f0f0f0f0
f0f0f0f0
f0f0f0f0
f0f0f0f0
f0f0f0f0
f0f0f0f0
f0f0f0f0
f0f0f0f0
f0f0f0f0
f0f0f0f0
f0f0f0f0
f0f0f0f0
endstream
endobj

xref
0 7
0000000000 65535 f
0000000010 00000 n
0000000079 00000 n
0000000128 00000 n
0000000189 00000 n
0000000357 00000 n
0000000440 00000 n
trailer
<<
/Size 7
/Root 1 0 R
>>
startxref
763
%%EOF